Reflecting modulator circuit comprising a negative impedance amplifier

ABSTRACT

A modulator circuit comprises a negative impedance amplifier which is operable such that a signal applied to the amplifier is reflected and amplified. Switching circuitry is provided for switching the impedance of the amplifier between two reflecting states such that the reflected and amplified signal is phase modulated. The impedances of the negative impedance amplifier are selected such that the phase of the reflected and amplified signal switches by substantially 180°. Preferably, the impedances of the negative impedance amplifier in the two reflecting states are selected such that the reflection gain of the amplifier in the two reflecting states is substantially the same such that the reflected and amplified signal is a binary phase shift keyed.

This invention relates to a modulator circuit and more especially tosuch a circuit for generating binary phase shift key modulation.

Modulation, which can be broadly defined as a time varying modificationof a signal to impart information thereto, is a crucial feature of thedesign of almost all radio based systems. An effective and well knownform of modulation for digital signals, is binary phase shift keying(BPSK). In BPSK one of the two digital states of information is impartedonto a carrier signal by modulating its phase to have two discretevalues which are generally separated by 180 degrees (π radians). Whilstsuch a modulation technique may be efficient it has not previously beenideally suited for applications where low cost and low power consumptionare paramount such as in tagging systems, since the known circuitry forgenerating BPSK is complex and consumes too much electrical power foroperation from a finite battery supply.

The present invention has arisen in an endeavour to provide a modulatorcircuit which at least in part overcomes the limitations of the knownmodulators and which is suitable for use in a tagging systems or otherapplications where low power consumption and circuit simplicity are ofimportance.

According to the present invention a modulator circuit comprises: anegative impedance amplifier operable such that a signal applied to theamplifier is reflected and amplified and switching means for switchingthe impedance of the amplifier between two reflecting states,characterised in that the impedances in the two reflecting states areselected such that the phase of the reflected and amplified signalswitches by substantially 180 degrees.

Preferably the impedances in the two reflecting states are selected suchthat the reflection gain of the amplifier in the two reflecting statesis substantially the same such that the reflected and amplified signalis a binary phase shift keyed.

Alternatively the impedances in the two reflecting states are selectedsuch that the reflection gain of the amplifier in the two reflectingstates is different and wherein said impedances are selected such thereflected and amplified signal is a substantially single sidebandsignal.

In a particularly preferred embodiment the negative impedance amplifiercomprises a transistor, such as for example a bipolar or field effecttransistor, and biasing means for biasing the transistor such as to actas a negative impedance amplifier. Such a modulator circuit is found tobe particularly advantageous since it in essence can comprise only asingle component. Furthermore, a negative impedance amplifier is capableof providing high gain at very low current, so its power consumption canaccordingly be very low of the order of a few micro-amps. Convenientlywhen using a transistor the switching means switches the biasing of thetransistor to switch the transistor between the two reflecting states.

Advantageously the modulator circuit further comprises an antenna forreceiving radiation and converting it to the signal applied to theamplifier and for radiating the reflected and amplified signal.

According to a second aspect of the invention there is provided ademodulator circuit for demodulating a Binary phase shift keyed signalwhich incorporates the modulator circuit described above.

According to a third aspect of the invention there is provided atransponder tag which incorporates the modulator circuit describedabove.

A modulator circuit in accordance with the invention will now bedescribed by way of example only with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic representation of a transponder circuit whichincorporates a modulator circuit in accordance with the invention;

FIG. 2 is a schematic representation of a de-spreader circuit for use ina spread spectrum communication system which incorporates the modulatorcircuit of FIG. 1; and

FIG. 3 is a schematic representation of a spread spectrum communicationsystem incorporating the modulator circuit of FIG. 1.

Referring to FIG. 1 there is shown a microwave frequency (2.45 GHz)pseudo passive transponder tag 1 for use in a tagging system whichincorporates a modulator circuit 2 in accordance with the invention. Thetag 1 comprises an antenna 4 which is connected to the modulator circuit2. The modulator circuit 2 comprises: a gallium arsenide (GaAs) fieldeffect transistor (FET) 6, impedance matching/feedback networks 8, 10,12 connected to a respective transistor terminal, a switchable currentsource 14 and a control circuit 16. The antenna 4, which for operationat microwave frequencies conveniently comprises a patch antenna, isconnected to the gate electrode 6 _(g) of the FET 6 via the matchingnetwork 8 which conveniently comprises a transmission line element. Thedrain electrode 6 _(d) of the FET 6 is connected to a positive supplyV_(supply) by the matching network 10. The source electrode 6 _(s)connected to ground via the matching network 12 and the switchablecurrent source 14. The current source 14 is controlled by the controlcircuit 16 via a control line 18.

In a known manner the FET 6 is biased by a biasing network whichcomprises the matching/feedback networks 8, 10, 12 such that it operatesin a linear relatively high gain region of its current/voltagecharacteristic. Conveniently each of the networks 8, 10, 12 comprises atransmission line element. The FET 6 thus amplifies and reflects anysignal appearing at its gate electrode 6 _(g) and therefore acts as anegative impedance amplifier. This being said, it will be appreciatedthat in most applications the impedance of the amplifier is primarilyresistive.

The magnitude of the negative impedance of the modulator circuit 2 isdependent on the drain/source current I_(ds) passing through thetransistor 6, and this current is determined by the switchable currentsource 14. The current source 14 is switchable between two selectedcurrents I_(ds1) and I_(ds2) in dependence upon control circuit 16. Forboth currents I_(ds1) and I_(dS2), the FET 6 operates as a negativeimpedance amplifier, though for each current the magnitude of itsnegative impedance is different.

In operation of the circuit 1 the antenna 4 receives and convertsmicrowave radiation 19 into an electrical signal which is applied viathe matching network 8 to the gate 6 _(g) of the FET 6. As describedabove the FET 6 acts as a negative impedance amplifier and theelectrical signal is reflected and amplified by the FET 6 andre-radiated as microwave radiation 20 from the antenna 4. In the case oftagging systems the microwave radiation 19 is an interrogating radiationsignal which can be a continuous wave or modulated wave signal. Toimpart information to the radiation 20 the control circuit 16 switchesbetween the two currents I_(ds1) and I_(ds2) such that the phase of theradiation 20 switches by 180 degrees. An important feature of theinvention is the selection of the magnitude of the negative impedance ofthe circuit 2 for the two currents I_(ds1) and I_(ds2). These areselected such that (i) the circuit has the same reflection gain for eachcurrent and (ii) the phase between the reflected and amplified signalfor the two currents is switched by 180 degrees. The reflection gain (indecibels dB) of the circuit 1 as seen looking toward the gate terminal 6_(g) is given by:

${gain} = {20\log{\frac{{Zn} - {Zo}}{{Zn} + {Zo}}}}$where Zo is the antenna impedance (or in the case where no antenna ispresent, it is the system impedance) and Zn is the input impedancepresented by the FET 6 (that is the negative impedance looking towardsthe gate 6 _(g)). For the embodiment shown in FIG. 1 the system/antennaimpedance is nominally 50 ohms and the value of the negative impedanceis switchable between −45 and −55.555 ohms for I_(ds1) and I_(ds2)respectively to give a reflection gain in each case of 25 dB. It is tobe noted that for these impedance values whilst the reflection gain isconstant, the phase of the reflected and amplified signals will bealtered by 180 degrees. This change of phase is indicated by the changeof the sign of the term (Zn−Zo)÷(Zn+Zo). Thus for the example of FIG. 1I_(ds1) is selected such that the FET 6 operates as a negative impedanceof −45 ohms and I_(ds2) is selected such that the FET 6 operates asnegative impedance of −55.555 ohms. It will be appreciated thereforethat the circuit 2 acts as a binary phase shift key reflectivemodulator. A particular advantage of the modulator circuit 2 is that itprovides a simple method of generating BPSK and offers the additionalbenefit that it also amplifies the signal which it is modulating. Due tothe circuit's simplicity it is ideally suited to tagging applicationswhere it further has the advantage that it is capable of operating atvery low currents (of the order of a few micro-amps) for an operatingfrequency of 2.4 GHz.

With different values for the respective impedances, both the magnitudeand phase of the reflected signal can be varied between the two states,such that a combination of amplitude modulation (AM) and phasemodulation (PM) can be applied. With an appropriate combination of thetwo forms of modulation the radiated signal 20 can be arranged to be asubstantially single sideband signal.

Referring to FIG. 2 there is shown a schematic of a de-spreader circuit21 for use in a spread spectrum communication system such as for exampleof the type used in a global positioning system. As is known in suchspread spectrum systems a carrier signal is modulated with a digitalcode, most often a pseudo random binary sequence (PRBS), to spread itsenergy spectra. Commonly the modulation used is BPSK. The circuit 21 isintended for de-spreading such spread spectrum radiation to recover theoriginal carrier signal and any modulation applied thereto. This isachieved by using the modulator 2 of FIG. 1 to apply a replica of thesequence used to generate the spread spectrum. It will be appreciatedthat the sequence applied by the circuit 21 is additionally in timesynchronisation with the generating sequence.

The circuit 21 comprises; an antenna 22 for receiving broad band spreadspectrum radiation 23, a broad pass-band filter 24, a narrow stop-bandfilter 25, a narrow pass-band filter 26 and a modulator circuit 2. Thebroad pass-band filter 24, narrow stop-band filter 25 and narrowpass-band filter 26 are connected in series and the output 28 of thenarrow pass-band filter 26 provides the output 28 of the circuit 21. Theantenna 22 is connected to the input 30 of the broad band filter 24. Themodulator circuit 2, which is identical to the circuit shown in FIG. 1,is connected to the interconnection 32 of the filters 25 and 26.

The reflective modulator circuit 2 has a gain of 20 dB in bothreflecting states. The reflecting state of the modulator 2 is controlledby a digital signal 34, which as described above is a replica of theoriginal sequence signal used to generate the broad band signal 23. Mosttypically the signal 34 is a PRBS signal.

In operation the broad band spread radiation 23 is received andconverted to an electrical signal by the antenna 22 and passes throughthe broad pass-band filter 24 and narrow stop-band filter 25. Thepass-band of the filter 24 defines the bandwidth of operation of thecircuit 21. The centre frequency of the stop-band filter 25 is selectedto correspond with the carrier frequency of the radiation 23 to blockany components at the carrier frequency. The filtered signal appearingat the output 32 a of the filter 25 is applied to both the input 32 b ofthe narrow pass-band filter 26 and to the input 32 c of the modulator 2.Due to the pass-band pass characteristic of the narrow band-pass filter26 the filtered signal is blocked by the filter 26. The filtered signalhowever appearing at the input 32 c of the modulator circuit 2 isde-modulated to produce an amplified version of the original carriersignal which is reflected back to the interconnection 32. The amplifiedcarrier signal, which is within the band pass characteristic of thenarrow pass-band filter 26, passes through substantially unattenuated tothe output 28. The demodulated signal is prevented from returning to theantenna 22 by the stop-band filter 25. The circuit 20 thus operates as ade-spreader circuit and is capable of operating at substantially lowercurrents than those which currently use digital techniques.

A further example of an application of the reflector modulator inaccordance with the invention is now described with reference to FIG. 3which is a schematic of a spread spectrum communication system 40 foruse in covert communications between a transmitter 42 and a hand heldradio receiver 44. As is known spreading the spectra of the transmittedsignal, and hence spreading the energy over a large frequency range,makes it more difficult for the signal to be detected by unauthorisedpersons and hence for such persons to determine the position of thetransmitting source.

Referring to FIG. 3 the communication system 40 comprises: a spreadspectrum transmitter 42 of a known type which generates a BPSK modulatedbroad band spread spectrum radiation 46, a reflective de-spreadingcircuit 48 and a hand held radio receiver 44. The de-spreader circuit 48is identical to the transponder circuit 1 of FIG. 1 in which the controlcircuit 16 switches the transistor 6 using an identical code to thatused by the transmitter 42 to generate the spread signal 46. Thede-spreader circuit 48 thus receives the broad band radiation 46 and inresponse radiates an amplified and de-spread narrow band radiation 50which represents the recovered carrier of the signal 46 and anymodulation applied thereto. The narrow band radiation 50 is detected bythe hand held radio receiver 44. The de-spreading circuit 48 ispreferably mounted at a high point such as on the side of a building 52or other structure such as a post or a tree. Since the radiation 46generated by the transmitter 42 is broad band this makes it difficultfor a direction finding receiver to locate the position of thetransmitter 42. Although such a direction finding receiver may be ableto locate the narrow band radiated emissions 50 from the de-spreadingcircuit 48 and hence determine its position, it will still be unable todetermine the position of the transmitter 42. In a preferredcommunication system a number of de-spreading circuits 48 (a second suchcircuit 48 a is shown in FIG. 3), each having a different modulationcode, are located at different physical locations. The transmitter 42 isoperable to switch between the different modulating codes duringcommunication with the hand held radio 44 such that differentde-spreading circuits 48 become activated. As a result the position fromwhich the narrow band radiation 50, 50 a originates will jump fromde-spreading circuit 48 to de-spreading circuit 48 a, thereby hamperingany attempt to locate the position of the de-spreading circuit.

It will be appreciated that modifications can be made to the circuitsdescribed which are still within the scope of the invention. For examplewhilst in the examples described the modulator circuit uses a fieldeffect transistor, which is much preferred for operation of microwavefrequencies, the negative impedance amplifier can be implemented indifferent ways, depending upon the required frequency of operation, suchas for example using a bi-polar transistor or other active devices.Furthermore the modulator circuit of the invention is not restricted tothe applications described and is suited for use in any applicationwhich requires BPSK modulation. The present invention resides in therealisation that binary phase shift key modulation can be achieved byusing a reflection amplifier and switching the circuit between tworeflecting states which preferably have the same reflection gain (thoughthis is not essential when single sideband operation is required), butwhich change the phase of the reflected signal by substantially 180degrees.

1. A modulator circuit, comprising: a negative impedance amplifieroperable for reflecting and amplifying a signal applied to theamplifier; and switching means for switching the amplifier between tworeflecting states having impedances in the two reflecting statesselected such that a phase of a reflected and amplified signal switchesby substantially 180°.
 2. The modulator circuit according to claim 1, inwhich the impedances in the two reflecting states are selected such thata reflection gain of the amplifier in the two reflecting states issubstantially the same and such that the reflected and amplified signalis a binary phase shift keyed signal.
 3. The modulator circuit accordingto claim 1, in which the impedances in the two reflecting states areselected such that a reflection gain of the amplifier in the tworeflecting states is different, and wherein the impedances are selectedsuch the reflected and amplified signal is a substantially singlesideband signal.
 4. The modulator circuit according to claim 1, in whichthe negative impedance amplifier comprises a transistor and a biasingmeans for biasing the transistor such as to act as the negativeimpedance amplifier.
 5. The modulator circuit according to claim 4, inwhich the switching means switches the biasing of the transistor toswitch the transistor between the two reflecting states.
 6. Themodulator circuit according to claim 1, further comprising an antennafor receiving and converting radiation to the signal applied to theamplifier, and for radiating the reflected and amplified signal.
 7. Themodulator circuit according to claim 4, in which the transistorcomprises a bipolar transistor.
 8. The modulator circuit according toclaim 4, in which the transistor comprises a field effect transistor. 9.A de-modulator circuit for de-modulating a binary phase shift keyedsignal, comprising: a modulator circuit including a negative impedanceamplifier operable for reflecting and amplifying a signal applied to theamplifier; and switching means for switching the amplifier between tworeflecting states having impedances in the two reflecting statesselected such that a phase of a reflected and amplified signal switchesby substantially 180°.
 10. A transponder tag, comprising: a modulatorcircuit including a negative impedance amplifier operable for reflectingand amplifying a signal applied to the amplifier; and switching meansfor switching the amplifier between two reflecting states havingimpedances in the two reflecting states selected such that a phase of areflected and amplified signal switches by substantially 180°.
 11. Atransponder tag, comprising: a negative impedance amplifier configuredto reflect a received signal; and a switchable biasing circuitconfigured to bias the amplifier in a first state wherein the amplifierreflects a first signal having a first phase, the biasing circuitfurther configured to bias the amplifier in a second state wherein theamplifier reflects a second signal having a second phase that differssubstantially from the first phase.
 12. The transponder tag according toclaim 11, wherein the negative impedance amplifier includes a transistorthat is biased by the switchable biasing circuit.
 13. The transpondertag according to claim 12, wherein the switchable biasing circuit isconfigured to modify a current passing through the transistor.
 14. Thetransponder tag according to claim 13, wherein the switchable biasingcircuit is configured to switch the current passing through thetransistor between two different currents.
 15. The transponder tagaccording to claim 14, wherein the two different currents are selectedbased on a desired phase difference of the first and second signals. 16.The transponder tag according to claim 14, wherein the two differentcurrents are selected based on a desired phase of the first and secondsignals.
 17. The transponder tag according to claim 13, wherein theswitchable biasing circuit includes a current source.
 18. Thetransponder tag according to claim 11, further comprising a controlcircuit configured to output a control signal that causes the switchablebiasing circuit to switch the amplifier between the first and secondstates.
 19. A method, comprising: providing a negative impedanceamplifier configured to reflect a received signal; biasing the amplifierto operate in a first state wherein the amplifier reflects a firstsignal having a first phase; and switching the biasing of the amplifierto operate in a second state wherein the amplifier reflects a secondsignal having a second phase that differs substantially from the firstphase.
 20. The method according to claim 19, wherein biasing of theamplifier includes modifying a current passing through the amplifier.21. The method according to claim 20, wherein the current passingthrough the amplifier is modified based on a desired phase difference ofthe first and second signals.
 22. The method according to claim 20,wherein the current passing through the amplifier is modified based on adesired phase of the first and second signals.